1. Field of the Invention
The present invention relates to an evaluation method for evaluating a pigment ink for ink-jet recording and a production method for producing the pigment ink for ink-jet recording.
2. Description of the Related Art
A pigment ink for ink-jet recording, which contains a pigment and water, has been hitherto widely used. A printed matter, which is printed by using the pigment ink, is excellent in the weather resistance and the water resistance. However, the pigment ink involves such a problem that the pigment ink is difficult to be fixed on a recording medium such as a printing paper sheet, and that the optical density of the printed matter is low. Further, the pigment ink also involves such a problem that the pigment tends to aggregate or coagulate, and the pigment ink is inferior in the dispersion stability. The inferior dispersion stability of the pigment means that the storage stability of the pigment ink itself is low. The inventors of this invention recognize that, other than the problem associated with the storage stability of the pigment ink, the inferior dispersion stability of the pigment also causes the following problem. In a case that the pigment ink inferior in the dispersion stability is used, for example, in an ink set together with a dye ink, the pigment ink is brought in contact with the dye ink before the printing at an ink jetting surface of an ink jet recording head in some cases. This situation happens, for example, when the ink jetting surface is wiped with a wiper so as to perform cleaning for the ink jetting surface. When the pigment ink which is inferior in the dispersion stability as described above and the dye ink are brought in contact with each other on the ink jetting surface, the pigment ink is affected, for example, by the Na+ counter ion contained in the dye ink and thus the pigment ink is easily aggregated, thereby clogging a nozzle hole in the ink jetting surface. This problem is very serious in an ink-jet head having a construction in which a plurality of color inks are jetted from a single jetting surface.
In order to solve the above problem associated with the optical density, there is proposed a pigment ink in which a water-soluble resin is blended (Japanese Patent Application Laid-open No. 2004-277562 and No. 2005-8725). In this pigment ink, by blending the water-soluble resin, the aggregation of the pigment on the recording medium is enhanced and the optical density of the printed matter is increased.
Further, in order to increase the optical density of the printed matter and to increase the dispersion stability of the pigment, there is provided a pigment ink containing acrylic resin (Japanese Patent Application Laid-open No. 2006-8897).
However, in the pigment ink in which the water-soluble resin is blended, the dispersion stability is lowered because the aggregation of the pigment is enhanced. Therefore, in a case that this pigment ink is used together with a dye ink in an ink jet recording apparatus, the pigment aggregates when the dye ink is brought in contact with the pigment ink and the aggregated matter is adhered to the nozzle of the ink-jet head, which in turn causes the discharge failure or unsatisfactory failure as a result. Further, also in the pigment ink in which the acrylic resin is blended, the problem of the aggregation, caused by the contact of the pigment ink with the dye ink before the printing, has not been resolved. Namely, among the conventional pigment inks, there is not such a pigment ink with which the high optical density of the printer matter is realized and which is capable of preventing the discharge failure caused by the contact of the pigment ink with the dye ink before the printing.
Conventionally, the dispersion stability of the pigment and the optical density of the printed matter, which relate to the pigment ink as described above, have been generally evaluated by performing a storage stability test and a printing test. Such an evaluation method requires a long test time and a complicated operation.
A method has been suggested, in which the dispersion stability of the pigment and the optical density of the printed matter are evaluated by the aggregation value (coagulation value) of the pigment ink with respect to sodium chloride (Japanese Patent Application Laid-open No. 2006-8897).
However, the evaluation of the dispersion stability in the evaluation method based on the aggregation value is an evaluation of the dispersion stability of the pigment in the pigment ink. With the evaluation method based on the aggregation value, it is impossible to evaluate the dispersion stability of the pigment for a case that the pigment ink is used in an ink set together with the dye ink and when the pigment ink is brought in contact with a dye ink before the printing. Further, it is desired that a method is developed which is capable of effectively evaluating the dispersion stability of the pigment when the pigment ink is jetted onto a paper surface. In particular, the optical density of the printed matter is affected by the dispersion stability of the pigment